1. Field of the Invention
This invention relates generally to microwave devices and in particular this invention relates to a tuning means for adjusting the capacitance of a resonator rod in a resonant microwave cavity.
Description of the Prior Art
2. Microwave filters and tunable microwave devices are generally known in the prior art. Various methods of tuning have been utilized and have met with varying degrees of success. Most of the tuning methods heretofore used have been costly to manufacture, assemble, tune, or maintain in a tuned condition.
Microwave devices using resonator rods are also well known in the prior art. The combination of the resonator rod and the resonant cavity presents a resonant element which may be an equivalent inductance or an inductance plus capacitance depending upon the length of the resonator rod. The effective length of the resonator rod may be varied by varying the capacitance between the open end of the rod and the cavity wall. One of the methods that has been used for adjusting the capacitance between the rod and the cavity has been to insert a tuning screw directly opposite the free end of the resonator rod. The capacitance is adjusted by adjusting the spacing between the end of the screw and the end of the rod. Such an arrangement limits the range of capacitance values that can be obtained, however. In addition, the capacitance values of such a device vary greatly with temperature due to the effect of the coefficients of thermal expansion. Another limitation of a cantilevered resonator rod is that under conditions of vibration the resonator rod acts as a tuning fork which results in an unstable resonant frequency.
Another method of providing variable capacitance to a resonator rod is to provide adjustable sides to the resonator cavity. This is accomplished by placing individual adjustable blocks opposite the open end of the resonator rod and adjusting the spacing therebetween for achieving the desired capacitance and the desired resonant frequency. Such an arrangement is costly to manufacture and provides a limited capacitance range and therefore limited tuning capability.
Generally, the methods used heretofore for adjusting the capacitance in a microwave device have been costly due to the high machining tolerances which are required for obtaining high rejection and low losses.